Aqueous cadmium removal with ferrate: Influencing factors, removal mechanism, and effect of coexisting ions

The aim of the work was to evaluate the potential of potassium ferrate (K 2 FeO 4 , Fe( VI )) removing Cd( II ) from aqueous solution and to primarily investigate the mechanism. Batch experiments were carried out to evaluate the influence of various factors including dosage of K 2 FeO 4 , solution pH , the initial concentration of Cd( II ), and coexisting ions. The results showed that increasing dosage of K 2 FeO 4 benefited the removal of Cd( II ) with the efficiency up to 98.97%. The enhanced removal occurring at higher pH could be attributed to less self‐decomposition of K 2 FeO 4 under alkaline conditions. The maximum specific removal capacity was 80 mg/(g K 2 FeO 4 ) under certain conditions. Surface properties of the Cd( II )‐containing precipitations were characterized, and the results showed that the weakly crystalline ferrihydrite derived from self‐decomposition of Fe( VI ) benefited the adsorption capacity, and Cd( II ) was removed via the adsorption of the hydrolysate nanoparticles. Moreover, the coexisting ions were found weakening the removal efficiency of Cd( II ) due to the restricted activity of K 2 FeO 4 and competition for adsorption sites of hydrolysate. The interaction between the hydrolyzed products and the adsorbed Cd( II ) was also weaken by compressing the thickness of the electric double layer because of the presence of coexisting ions. Practitioner points Removing Cd(II) from aqueous solution as high as 98.97%. Removal efficiencies of Fe(VI) relied on chemical dose, pH and contaminant concentration. Co‐exiting ions weaken the removal efficiency, such as Ca 2+ and SO 4 2‐ as well as ionic strength.